USB (Universal Serial Bus) is a specification to establish communication between devices and a host controller (such as a personal computer). Universal Serial Bus (USB) is intended to replace many varieties of serial and parallel ports. USB devices can connect computer peripherals such as mice, keyboards, digital cameras, printers, personal media players, flash drives, and external hard drives. For many of those devices, USB has become the standard connection method. USB was designed for personal computers, but it has become commonplace on other devices such as Smartphones, Personal Digital Assistants (PDAs) and video game consoles, and as a power cord between a device and an AC adapter plugged into a wall plug for charging. As of 2008, there were about two billion USB devices sold per year, and approximately six billion total sold.
USB 1.0 and 1.1 specify a “full speed” data rate of 12 Mbit/s. Universal Serial Bus 2.0 (USB2) adds a higher maximum bandwidth of 480 Mbit/s (called “Hi-Speed” or “high speed”).
FIG. 1 and FIG. 2 show a possible implementation of full speed and high speed driver, respectfully. For a full speed driver, the USB specification requirement for the output impedance from a high speed output (Vo+ or Vo−) to ground or to the supply voltage, Vdd, is 28<Zo<44 ohms. In FIG. 1, the output impedance from the output (Vo+ or Vo−) with respect to the supply voltage, Vdd, is the impedance of the resistor 102 or 104 plus the impedance of the transistor 106 or 108. The output impedance from the output (Vo+ or Vo−) with respect to ground is the impedance of the actual resistor 102 or 104, plus the impedance of the transistor 110 or 112. The resistor value is the major contribution to the output impedance and the transistors 106, 108, 110 or 112 are sized with respect to the impedance of the resistor to meet the output impedance requirement of the USB specification.
Looking at FIG. 2, in the high speed driver, the specification requirement for output impedance from each output (Vo+ and Vo−) respect to ground is 40.5<Zo<49.5 ohms. The output impedance (Zo) with respect to ground is the impedance of the resistors 202 or 204 plus the output impedance of the transistor 206 or 208. Since high speed drivers have such a tight impedance requirement, a fix resistor (˜40 ohms) and a feedback system can be used to control the impedance of transistors 206 and 208.
In a USB2 upstream device, When the full speed driver is part of high speed capable transceiver, the impedance of each of the drivers (full and high speed) must be 40.5<Zo<49.5 ohms.